Fluid delivery is needed in a variety of applications, e.g. medicine delivery in medical devices, fuel injection in internal combustion engines, and reductant delivery in engine exhaust gas treatment systems. To accurately deliver required amount of fluid, in a fluid delivery apparatus, normally fluid needs to be metered, and the metering methods can be either a pre-metering method, in which the amount of the fluid to be delivered is metered before delivered, or a common rail method, in which the fluid is contained in a common rail with its pressure controlled, and the metering is achieved by controlling the opening time of a nozzle fluidly connected to the common rail during delivery. In the pre-metering method, since the fluid pressure is not controlled, without an assistant means, e.g. compressed air, it is difficult to satisfy both of delivery rate and atomization requirements, which need the fluid pressure be controlled in a certain range. Also, limited to metering accuracy and speed, it is also difficult for a pre-metering method to rapidly change fluid delivery rate. In the common rail method, the pressure of the common rail is controlled to a constant value. Thereby it is relatively easy to achieve both delivery rate and atomization requirements. And since the fluid delivery rate is determined by the opening time of the nozzle, with a high common rail pressure and fast-response nozzle control, fluid delivery rate can be changed quickly. However, in the common rail method, if a rigid common rail is used, the suddenly opening of the nozzle may create a pressure spike, causing problems in common rail pressure control and fluid delivery rate control. To solve this problem, normally a hydraulic buffer is used with the common rail for damping pressure change. The hydraulic buffer and common rail can be the same device and two types of hydraulic buffers are normally used. One type hydraulic buffer is a spring buffer that has a spring inside providing a linear relation between pressure change and volume change and the proportional coefficient is determined by the spring constant. The other type hydraulic buffer has air trapped inside. The volume change of the trapped air damps the pressure change. In addition to smoothing pressure change, the hydraulic buffer can also provide pressing force when fluid supply to the hydraulic buffer interrupts, e.g. when a membrane pump or an air-driven pump is used in supplying fluid.
The fluid to be delivered may have certain solubility for the trapped air in a hydraulic buffer, and normally the higher the pressure, the higher the solubility is. If such a fluid is delivered, the trapped air may be brought out by the fluid, resulting in poor delivery performance. One method for solving this problem is refilling air when the trapped air is exhausted. However, the air refilling needs to be carefully controlled, since too much refilling air would enter the fluid to be delivered and cause delivery rate issues. Controlling air refill without increasing control system complexity is a challenging problem.
Control system complexity is also a concern in pump controls, especially in the control of an air-driven pump. When an air-driven pump is used for supplying fluid, a two-stroke control can be used in controlling the pressure in the buffer, i.e., in a suction stroke when the compressed air in the pump is released, fluid flows into the pump and in a pressing stroke when compressed air goes in the pump, a pressure is built up to provide a driving force for fluid delivery. In switching between these two strokes, the fluid level in the pump and the buffer is normally used in triggering the change of the strokes. However positioning fluid level sensors in the pump and the buffer will increase system complexity and fluid sloshing may introduce errors in fluid level sensing and cause control problems. It is desirable to use as few sensors as possible in the stroke control.
Decreasing control system complexity also benefits system diagnosis. In applications that require high reliability, such as that in medical instruments and engines, the fluid delivery performance needs to be monitored, and a fault is reported when an anomaly is detected to avoid failures in the fluid delivery apparatus from causing other issues. For example, engine controls require on-board diagnostics (OBD), which need to report a fault when a problem is detected. On one hand, to better monitor the fluid delivery performance, more sensing information is beneficial. On the other hand, however, more sensors may create more issues and hence need more diagnostics for themselves. Therefore, it is better obtaining more information from fewer sensors in detecting system issues. And it would be more desirable to use one sensor for multiple sensings.
In addressing the issues and requirements mentioned above, the present invention provides a fluid delivery apparatus with a diagnostic controller that detects issues in delivering fluid with a single pressure sensor positioned in the buffer of the fluid delivery apparatus. The present invention also provides a controller for an air driven pump switching in between a suction stroke and a pressing stroke, and a controller for refilling trapped air in a buffer and a tank fluid level sensing means using the sensing value obtained from the pressure sensor. Additionally, based on the pressure sensor, the present invention further provides a sensing means for detecting fluid level in a tank of the fluid delivery apparatus.